Continuous gravimetric feeders or conveyors for transporting bulk material (e.g., crushed coal) from a feed hopper to a receiving bin or metal ore furnace are generally known in the art. Such feeders typically include a conveyor belt that rotates about a pair of oppositely disposed pulleys and a weighing platform for the continuous weighing of bulk material as it passes over a predefined weigh span section. In particular, an upper strand of the belt may be supported by numerous support rollers that are spaced apart a predetermined amount. It is the spacing between any two support rollers having a weigh roller disposed therebetween that defines the length of the weigh span section. As bulk material passes over the weigh span section of a gravimetric feeder, a microprocessor connected to the weigh span section and belt drive (motor) determines the feed rate, i.e., net weight per unit of time, of bulk material carried by the conveyor belt. Thus, by monitoring the feed rate, the speed of the feeder may be controlled to conform with the momentary bulk material delivery requirements. Conveyors equipped with a belt scale may also use microprocessors connected to the weigh platform which integrate the measured weight as bulk material passes over the weigh span. Conveyors of this type are used to transport bulk materials and not to control the rate of delivery.
Typically, the weigh assembly includes a weigh roller for continuously determining the weight of bulk material within the weigh span section of the conveyor. The weigh roller is supported between two support rollers by a piezoelectric type or strain gauge type load cell transducer that produces an electric signal corresponding to the gross weight of bulk material and the tare weight presently disposed within the weigh span section. As bulk material passes over the weigh span section, a microprocessor connected to the assembly receives gross weight information that is the sum of bulk material weight and the tare of the weigh platform and processes this information to calibrate and control the operation of the weigh assembly and gravimetric feeder.
Conventional gravimetric feeders and conveyors have been calibrated using a static reference load. The calibration is conducted by first compensating for tare, then applying a static load consisting of a precisely known weight or weights to an appropriate member of the weigh span section, then calibrating the microprocessor control to display the reference weight. This method of calibration may not always calibrate the weigh span section correctly, and errors can arise due to the reference weight or weights inability to induce a change in belt tension and deflection, such as belt sag, or duplicate structure deflection that otherwise occurs with a material load on the conveyor belt of a gravimetric feeder or conveyor. Also, errors that may arise from bearing friction of the weigh rollers when loaded are ignored during calibration with hanging weight assemblies.
Calibration of gravimetric feeders and conveyors has been verified or certified by material testing. In fact, when questions arise for the above calibration method regarding delivery error, a material test is usually performed to ultimately check its accuracy. Verification is accomplished by passing a quantity of bulk material that the feeder normally conveys, over the weigh span section and comparing the weight of bulk material determined by the test to the reference weight determined either before or after the test on a large precision batch scale. Due to its cost in labor and in down-time of the equipment, a material test is usually used as a last resort. Further, there are serious potential errors that can occur if the test is not carried out with conspicuous attention to detail. These include reduction of material and moisture loss during transport to the scale, materials left in hoppers due to poor flow conditions, and improper consideration of the tare of the transport means.
In addition to material testing, test chains have been used as a reference load to test gravimetric feeders and conveyors. Because test chains provide a reasonable simulation of the loading of the weigh span section, they have been used to indicate when the scale is in need of re-calibration or repair. Initial set-up has usually been accomplished after a material test has been conducted whereby the chains are applied across the weigh span section, noting their position with respect to a fixed reference point on the feeder or conveyor frame and recording the indicated weight. Periodically thereafter, calibration is checked by placing the test chain on the weigh span section, realigning them to the fixed reference position on the feeder or conveyor frame and recording the new weight reading. If the subsequent weight reading of the test chain deviates beyond a particular threshold tolerance, the weigh assembly must be re-calibrated again with another pre-weighed quantity of bulk material.
Test chains are not presently used for direct calibration or as weight reference by conventional systems. It is generally perceived in the industry that test chains produce results that are erratic and sensitive to their placement on the conveyor belt. Because of the perceived, unreliable nature of chain testing due to position sensitivity, test chains have only been used as a reference on weigh span sections for detecting a change in scale performance. As a consequence, the material test calibration described above has been the only method used to identify problems with the weigh span section of a feeder. Thus, in spite of the inconvenience and cost of a material test calibration, such calibration has generally remained in the industry as the preferred method of calibration.
Chain testing may load the weigh span section of a gravimetric feeder in a similar manner as bulk material, but it does simulate the changes in belt tension caused by material loading. In addition, chain testing has significant and demonstrable position instability.
Accordingly, the present invention is a calibration checking system that overcomes the problems associated with the above described conventional systems of material test calibration and reference weights.
The present invention provides a test chain having particular chain pitch that is determined based on the weigh span pitch of its respective conveyor or feeder. Also the gravimetric feeder of the calibration checking system according to the present invention has a particular structural geometry and the weigh assembly has a means for compensating for belt tension to provide a highly accurate and reliable system for checking the calibration of the gravimetric feeder or belt scale. In fact, the calibration checking system of the present invention is capable of obtaining readings that match the known weight of the test chain with an accuracy approaching .+-.0.1%, whereas conventional calibration checking systems methods ignore the true weight of the chains and compare a currently measured indicted weight to an old recorded indicated weight.